A display element including a current drive type light-emitting unit, and a display device including the display element are known. For example, a display element including a light-emitting unit utilizing electroluminescence of an organic material (hereinafter may be simply referred to as an organic EL display element) is noted as a display element capable of high luminance light emission by low voltage direct current drive.
Similarly to a liquid crystal display device, for example, in a display device including the organic EL display element, a simple matrix system and an active matrix system are known as a drive system. The active matrix system has a disadvantage that its structure is complicated, but has an advantage that a luminance of an image can be increased, for example. The organic EL display element driven by the active matrix system includes a light-emitting unit including an organic layer including a light emitting layer and the like, and also includes a drive circuit for driving the light-emitting unit.
As a circuit for driving the current drive type light-emitting unit, for example, a drive circuit including two transistors and one capacitor (referred to as a 2Tr/1C drive circuit) is known from, for example, Japanese Patent Application Laid-Open No. 2007-310311 (Patent Document 1). As illustrated in FIG. 1, the 2Tr/1C drive circuit includes two transistors, a write transistor TRW and a drive transistor TRD, and one capacitor CS. Here, a gate electrode of the drive transistor TRD configures a first node ND1, and a source/drain region connected to a light-emitting unit ELP configures a second node ND2.
The capacitor CS is used to hold a voltage of the gate electrode (so-called gate-source voltage) with respect to a source region of the drive transistor TRD. Here, a threshold voltage Vth of the drive transistor TRD varies for each display element. Therefore, if the voltage held by the capacitor CS is a voltage reflecting only a video signal voltage, luminance unevenness occurs due to variation in the threshold voltage Vth. For this reason, the drive circuit is driven such that the voltage held by the capacitor CS is a voltage reflecting the video signal voltage and the threshold voltage Vth.
If the write transistor TRW is set in a non-conductive state in a state where the voltage reflecting the video signal voltage and the threshold voltage Vth is held by the capacitor CS, a current flows via the drive transistor, and the light-emitting unit ELP emits light. A voltage of an anode electrode of the light-emitting unit ELP, in other words, a voltage of the second node ND2 increases with light emission of the light-emitting unit ELP. At this time, the voltage of the gate electrode of the drive transistor TRD, in other words, the voltage of the first node ND1, increases due to bootstrap operation. If ideal bootstrap operation occurs, the gate-source voltage is kept before and after the bootstrap operation, so that the luminance unevenness does not occur due to the variation in the threshold voltage Vth of the drive transistor.